The present invention relates to a magnetic disk unit and a manufacturing method of a carriage thereof and, more particularly, to a method of manufacturing a low thermal deformation carriage structure having minimal thermal off-tracking and a high reliability, and to a magnetic disk unit utilizing the carriage structure.
As disclosed in Japanese Patent Unexamined Publication No. 63-188878, a conventional apparatus comprises actuator arms and the like which are integrally formed, with support arms and the like to which a plurality of magnetic heads (hereinafter, heads) are secured by screw-fasteners or adhesive material being attached to the distal ends thereof. Thus, thermal deformation of the joint portions due to a change of temperature can be reduced so as to prevent a positioning error of the heads.
Further, this one-piece carriage is provided with a coil for a voice coil motor which actuates the carriage.
An apparatus of the aforementioned is disclosed in, for U.S. Pat. No. 4,796,122, and Japanese Utility Model Unexamined Publication Nos. 64-12264, 1-140669, 1-140670.
Next, in relation to a structure of an Al-Si alloy as a material of the one-piece carriage, a method of making eutectic-crystal Si particles finer by heating and processing to thereby improve cutting capability of a work piece is disclosed in Japanese Patent Unexamined Publication No. 50-57906.
In the conventional technique described above, the one-piece carriage is used to solve the vibration problem of a plurality of support arms for supporting the heads. Particularly, in order to reduce thermal deformation due to bonding of dissimilar metals caused between the support arms and the one-piece carriage in response to a change of temperature, the one-piece carriage is made of an alloy material (e.g., an Al-Si alloy) such that the coefficient of thermal expansion of the carriage is substantially equal to that of the support arms. Additionally, a die casting method, excellent in productivity, is employed as a method of forming the one-piece carriage.
In the conventional technique, the one-piece carriage is arranged in such a manner that guide arms whose proximal ends are integrally held by a common guide-arm retaining portion are juxtaposed to be separate from one another in the axial direction of the guide-arm retaining portion. However, sufficient consideration is not given to prevention of thermal deformation force, which is generated between bearings attached to a rotary shaft and bearing retainers of the carriage due to the temperature change, from being transmitted to the outermost guide arm. Namely, a structure for minimizing the deformation is not taken into account. The conventional technique involves a problem in further decreasing an amount of thermal off-tracking of the magnetic disk unit of a dedicated servo system. The amount of thermal off-tracking in this case is a difference between amounts of thermal deformation at the distal end of a head of a guide arm for data and at the distal end of a head of a servo guide arm due to temperature increases and decreases, which will be described more fully hereinbelow.
Moreover, in the conventional technique, a coil portion and the one-piece carriage are directly combined. Usually, the coil portion and the one-piece carriage are made of metals having different coefficients of thermal expansion. When an electric current flows to the coil portion, heat is generated in it so as to induce non-uniform thermal deformation between the metals of these members. As a result, an error in a relative positional relationship results between the heads provided at the distal ends of the support arms and magnetic disks, thereby resulting in of thermal off-tracking.
In the one-piece carriage, however, a guide arm portion, a side of the guide-arm retaining portion which is opposite to the side which faces the guide arm portion (hereinafter guide-arm back portion), the portions where a flexible printed circuitboard is attached (hereinafter FPC holders) having different thickness so that there is a difference in cooling speeds at the time of die casting production. Moreover, the structures and compositions (sizes of primary crystals and eutectic crystals of Si and amounts of Si) are different, and the constants of the materials such as, for example, thermal expansion coefficients and the like being different. Therefore, thermal expansion of each portion in the one-piece carriage is non-uniform, and amounts of thermal expansion of the respective portions differ, resulting in a positioning accuracy problem of the heads when the dedicated servo system is employed.